This method provides a technique for on-vehicle NOX adsorber desulfation especially for use in NOX adsorber catalyst equipped diesel vehicles employing a multi-leg exhaust flow path.
New emission reduction standards for lean burn heavy-duty diesel engines are to be implemented starting in model year 2007. The new standards will require catalysts and systems that can suppress the emission of oxides of nitrogen (NOX) from these engines into the atmosphere. Current NOX adsorber catalyst formulations typically contain a combination of one or more of the following substances: alkali metals such as potassium (K), sodium (Na), lithium (Li) and cesium (Cs); alkali earth metals such as barium (Ba) and calcium (Ca); rare earth metals such as lanthanum (La) and yttrium (Y); and precious metals such as platinum (Pt) and rhodium (Rh). The precious metals in the catalyst wash coat oxidize NO and NO2 to nitrate ion (NO3xe2x88x92) and the nitrate ion is subsequently absorbed by the NOX adsorbent (alkali metals, alkali earth metals and rare earth metals) to form stable nitrates. These nitrate ions are then desorbed in a rich exhaust environment (lambda less than  less than 1) at normal engine operating temperatures and reduced over precious metal sites to diatomic nitrogen.
It has been found in development testing that the NOX storage and reduction capacity of the adsorber decreases over time. The main mechanisms responsible for the decrease in adsorber NOX storage and reduction capacity are thermal degradation of the adsorber wash coat and poisoning due to the presence of sulfur in diesel fuel. The sulfur is first oxidized during combustion, forming SO2. The SO2 is then further oxidized to SO3 and sulfate ion (SO42xe2x88x92) via the reaction with O2xe2x88x92 or O2xe2x88x92on the surface of the platinum in the adsorber wash coat. The sulfate ion is then adsorbed by the NOX adsorbent (alkali metals, alkali earth metals, and rare earth metals) to form stable sulfates (for example BaSO4), reducing the number of sites available for NOX adsorption. These sulfates have a higher binding affinity for alkali/alkali-earth/rare earth metals than nitrates, thus requiring temperatures that are much higher than those present in typical diesel exhaust to be desorbed. Higher temperatures, in conjunction with a rich exhaust environment (lambda less than  less than 1) are required to remove the sulfate ion from the NOX adsorbent. The process of removal of sulfates from NOX adsorbers will be referred to herein as desulfation.
The present invention provides a method and apparatus for desulfating NOX adsorber catalysts in a multi exhaust path flow system utilizing in-exhaust fuel injection and exhaust flow bypass. This method minimizes temperature extremes on the surface of the NOX adsorber, while achieving the precise temperatures required for desulfation. Thus, overall thermal degradation of the adsorber catalyst due to high temperatures is kept to a minimum or ultimately eliminated. This method allows the sulfates to desorb from the adsorber catalyst, as H2S and SO2, at exhaust lambda values  less than 1.
The present invention generates an exotherm on particulates accumulated in a trap upstream of a NOX adsorber, convectively transfers heat to the NOX adsorber, and minimizes local temperature extremes on the surface of the NOX adsorber. This, in turn, reduces the chances for thermal damage, e.g., deactivation of the adsorber NOX storage and reduction function due to sintering and migration of the wash coat into the catalyst substrate (i.e., migration of alkali, alkali earth, and rare earth metals). The present invention also allows better control over the desulfation and does not affect the drivability of the vehicle, as has been reported in connection with single leg systems.
More specifically, the present invention splits an exhaust stream into two or more paths or legs, each leg of the system containing of a NOX adsorber and a particulate trap, preferably a catalyzed diesel particulate filter (hereinafter xe2x80x9cCDPFxe2x80x9d), upstream of the NOX adsorber. While desulfating one of the multiple flow paths, the desulfating path is bypassed so that only a very small fraction of the exhaust flows through the desulfating NOX adsorber. This flow is due to incomplete sealing of the exhaust brake used to shut flow off to the bypassed leg. The incorporation of the perpendicular exhaust bypass loop allows controlled addition of exhaust from the adsorbing leg to the desulfation leg. This addition of exhaust allows for control over the mass of oxygen in the desulfating leg. Reductant is added via secondary fuel injection directly into the desulfating leg, upstream of the CDPF. The oxygen causes an exothermic oxidation of the reductant across the CDPF. The extent of the exotherm is determined by the lambda value in the desulfating leg, which is a product of the amount of reductant and oxygen present in the leg. Lambda can also be defined as the ratio of actual oxygen concentration to the oxygen concentration required for stoichiometry. The exotherm causes a rise in the CDPF temperature which is monitored. In pilot plant experimentation the CDPF temperature was measured by six thermocouples inserted along the CDPF horizontally. The heat is convectively transferred from the CDPF to the NOX adsorber catalysts by manipulation of the exhaust bypass flow rate from the adsorbing leg to the desulfating leg. Heat transfer can also occur without using the perpendicular exhaust bypass loop by momentarily opening the desulfating leg to full exhaust flow and then closing off the flow once the heat has transferred. Once the adsorber is heated to the desired temperature, reductant is added to facilitate sulfur release. The perpendicular exhaust bypass loop flow is controlled to maintain an exotherm across the CDPF and to allow heat transfer from the CDPF to the NOX adsorber to maintain the desired desulfation temperature.
Accordingly, the present invention provides a method for treating an exhaust gas stream which is in a lean state, fuel-lean of stoichiometric, and which contains NOX and SO2. The method includes splitting the exhaust gas stream into major and minor exhaust gas portions for flow through at least first and second separate flow paths, each of the flow paths containing a particulate trap and, downstream of the particulate trap, a NOX adsorber containing a NOX oxidation catalyst and a nitrate adsorbent. The major portion of the exhaust gas is passed in the lean state, for a first period of operation, along at least one of the flow paths and through, in succession, the particulate trap and the NOX adsorber to convert NOX to nitrate, to convert the SO2 to sulfate and to adsorb the nitrate and the sulfate on the nitrate adsorbent. Temperatures in the particulate trap and the NOX adsorber are monitored. After the first period of operation, the flows of the exhaust gas portions are switched so that the one flow path receives the minor exhaust gas portion for a second period of time and, during at least a part of the second period of time, fuel is introduced into the one flow path, upstream of the particulate trap, for combustion in the particulate trap to produce a fuel-rich, reducing exhaust flow. During that same second period of operation, a bypass portion of the exhaust gas is diverted from another flow path at a point upstream of the particulate trap and is introduced into the one flow path also upstream of its particulate trap. When the temperature of the particulate trap in the one flow path reaches a first predetermined temperature, heat of fuel thereto is discontinued and exhaust gas flow is increased to transfer heat from the particulate trap to the NOX adsorber, to raise the temperature of the NOX adsorber to a second predetermined temperature for desulfation. The major and minor exhaust gas portions are periodically switched between flow paths so that a second period of operation is effected in one flow path while a first period of operation is effected in at least one other flow path.
Engine speed and/or engine load may be monitored and the amount of fuel introduced and bypass flow exhaust introduced, during the second period of time, may be set in accordance with the determined engine speed and/or engine load.
The method of the present invention may further include sensing the NOX concentration exiting each NOX adsorber and, responsive to that sensed concentration exceeding a predetermined value, switching the flows of exhaust gas portions so that a flow path to be subjected to denitration receives the minor exhaust gas portion. Fuel is then introduced into the minor exhaust gas portion to create a reducing atmosphere for reduction of nitrates adsorbed on the nitrate adsorbent, to form molecular nitrogen gas.
The present invention is also embodied in an apparatus for treating an exhaust gas stream having the aforementioned characteristics. The apparatus includes plural exhaust flow conduits for respectively receiving portions of exhaust gas from an internal combustion engine. Each exhaust gas conduit includes a particulate trap and, downstream of the particulate trap, a NOX adsorber containing a NOX oxidation catalyst. The apparatus further includes temperature sensors for monitoring temperatures of the particulate traps and the NOX adsorbers. A bypass line connects each exhaust flow conduit with at least one other exhaust flow conduit, at points upstream of the particulate traps. A regulating valve is provided in the bypass line and a fuel injector is provided in each of the exhaust flow conduits, upstream of the particulate trap. A controller controls the regulating valve and the fuel injector for an exhaust flow path for desulfation, responsive to the sensed temperatures and engine speed and/or engine load.